Why distilled water or buffer added to the stain after fixation?

Distilled water or buffer is added to the stain after fixation to ensure optimal staining conditions and maintain the integrity of the sample. It helps to dilute the stain, allowing for more uniform distribution and preventing excessive staining that could obscure results. Additionally, using a buffer can help maintain the pH, which is crucial for the stability of the stain and the interaction between the stain and the tissue or cells. This step enhances the clarity and contrast of the stained sample for better visualization under a microscope.